Mac Local Procedure Parameters: what changed
Revision 0 to 1. Removed lines have a minus sign; added lines have a plus sign.
Title: Mac Local Procedure Parameters
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| 1 | + | *(macOS only)* | |
| 2 | + | ||
| 3 | + | > **Note:** This is a proposal | |
| 4 | + | ||
| 5 | + | In procedure A, a reference to a local or global procedure C is to be passed as a parameter to a procedure B. Procedure B might be declared in another unit, unaware of possible type of callers. The body of B will in turn call procedure C. If procedure C is local, it can now access variables local to A, if global, it can't. Procedure B might also pass the parameter further, but is not allowed to store the parameter in a variable. | |
| 6 | + | ||
| 7 | + | The procedure parameters can be implemented as a conceptual record with one procedure pointer and one local scope pointer. | |
| 8 | + | ||
| 9 | + | When passing local procedures, the local scope pointer will point to the local variables, when passing a global procedure, the local scope pointer is set to nil. | |
| 10 | + | ||
| 11 | + | The tricky problem is how B should call C. If the local scope pointer is nil, C should be called as a global procedure, if not nil it should be called as a local procedure, that is along with the local scope pointer. | |
| 12 | + | ||
| 13 | + | By putting the local scope pointer last, the procedure calling code chunk can be the same for local and global calls. A runtime check checks whether the local scope pointer is not nil, and if so pushes it. | |
| 14 | + | ||
| 15 | + | All three procedures A, B and C have to be declared in MacPas units for this to work, otherwise there is a compilation error. | |
| 16 | + | ||
| 17 | + | In the future, for some register based calling conventions, if the local scope pointer is put in a register which otherwise would not be in use, the runtime check can be optimized away. (for PowerPC, if the local scope pointer happens to be allocated in one of r3 - r10) | |
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| 19 | + | Also, cdecl for local procedure calls should not be allowed. | |
| 20 | + | ||
| 21 | + | --- | |
| 22 | + | *From the Free Pascal wiki, page "Mac Local Procedure Parameters", as it stood on 2019-10-17 after a change by Trev, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).* | |
| 23 | + | Category: Proposals | |
| 24 | + | Category: macOS |